Investigating the mechanism by which thalamocortical projections reach the cerebral cortex

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Abstract

This thesis provides insights into the mechanism by which thalamocortical axons
(TCAs) approach the cortex from their origin in the thalamus. Previous studies
suggested that the reciprocal projections from the prethalamus and the ventral
telencephalon guide TCAs to descend through the prethalamus and cross the
diencephalic-telencephalic boundary (DTB), after which TCAs navigate through
permissive corridor cells in the ventral telencephalon and cross the pallial-subpallial
boundary (PSPB) before reaching their final targets in the cortex. The ‘Handshake
Hypothesis’ proposed that pioneer axons from cortical preplate neurons guide TCAs
into corresponding cortical areas. However, there is a lack of convincing evidence on
whether TCAs need any guidance to cross the PSPB.
In the current study, Adenomatous polyposis (Apc) gene is conditionally deleted
from the cortex, by using Emx1Cre-APCloxP recombination technology. Apc is widely
expressed in the nervous system including the cortical plate of the cortex and
regulates axonal growth and neuronal differentiation. Deleting Apc may block neurite
extension and/or affect the formation of attractive or repulsive cues in the cortex. By
using DiI tracing as well as L1 immunohistochemistry techniques, I showed that in
the Apc mutants cortical axons are absent and that TCAs initially navigate into the
ventral telencephalon normally but fail to complete their journey into the cortex.
They stop as they approach the PSPB, although the PSPB doesn’t seem to be directly
affected by the mutation of Apc in the cortex. Additionally, Ig-Nrg1 (Neuregulin-1),
the secreted protein that was suggested to play long-range roles in attracting TCAs
towards the cortex, is present in the Apc mutant. This implies that Ig-Nrg1 is not
sufficient for guiding TCAs into the cortex, and that additional guidance factors are
needed. Moreover, my in vitro explant culture experiments show that the mutant
cortex neither repel nor inhibit thalamic axonal outgrowth, indicating that the failure
of TCAs in reaching the cortex is not due to the change of repulsive cues secreted by
the mutant cortex. It rather indicates that the guidance factors for TCAs are likely to
function through cell-cell contact mediated mechanisms. The Apc mutant cortex
lacks these guidance factors, which might be the cortical axons. In conclusion, my data reveal a choice point for TCAs at the PSPB. Guidance factors
from the cortex are needed for TCAs to cross the PSPB, which are absent in the Apc
mutant. TCAs may need the direct contact with cortical axons and use them as an
axonal scaffold to navigate into the cerebral cortex.